Troops loyal to Libya's new leaders maintained strong momentum Tuesday against pro-Moammar Gadhafi holdouts as they planned an assault on one loyalist city and moved into another without resistance.

A National Transitional Council official said fighters were planning a major assault on Bani Walid, a northern town still held by Gadhafi loyalists. "Our fighters are planning a massive attack today to bring the siege to an end," Abdallah Kenshil said.

Meanwhile, NTC fighters moved into Sabha, a southern town long regarded as a pro-Gadhafi stronghold, and have met no resistance so far, according to a CNN team that accompanied the forces.

This comes as an audio message purported to be from the deposed Libyan leader said the Gadhafi political system represented the people's will and could not be overthrown.

The message -- broadcast Tuesday on Al-Rai TV, a Syrian-based, pro-Gadhafi television station -- said Gadhafi supporters are mocking the belief that the longtime ruler and his government can be overthrown.

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"This wonderful system is impossible to be overthrown," the speaker said. "There is a regime that is being forced on the Libyan people by air and naval bombardment. This is a joke, and its legitimacy is hanging in the air, tied up with bombs and NATO planes that will not last."

In Bani Walid, forces loyal to the deposed Libyan leader are creating a humanitarian disaster, the NTC said.

Col. Ahmed Bani told reporters Monday that Gadhafi forces are robbing food stores, leaving civilian residents to starve. He also charged that Gadhafi loyalists are shooting everyone trying to join the revolution, including men, women and children. "They are carrying out mass killings," said Bani, the NTC military spokesman.

"This proves they are trying to destroy the town before it is liberated," he added, calling the Gadhafi forces "criminals" and "killers."

Asked by CNN why, if there is such a humanitarian disaster, NTC forces do not immediately enter the city, the colonel said the problem is "geography." There are tactics the forces must follow to reduce casualties, he said.

Bani Walid is surrounded by anti-Gadhafi forces, he said.

Asked to comment on assertions by Gadhafi spokesman Musa Ibrahim that loyalist forces in Bani Walid had captured a group of 17 mercenaries fighting for the NTC, including some British, Qatari, French and Asians, the spokesman dismissed the claim, saying, "Don't listen to him, he's from another planet."

The rebel spokesman's remarks at a news conference came after rebel fighters suffered a sizable blow. More than 20 were killed in the northern city of Sirte on Sunday, the country's transitional government said Monday.

Another 31 fighters were injured, said Abdel Rahman Busin, an NTC spokesman.

Sirte, Gadhafi's hometown, is among a handful of pro-Gadhafi strongholds left in the country.

"The injured revolutionaries in Sirte have all been hit with RPGs (rocket-propelled grenades) fired from areas congested with civilians where Gadhafi loyalists are hiding," Adel Ghulaek, spokesman for the NTC in Misrata, said Monday. "Our men are not even firing back because they do not want to kill any innocent people. Two helicopters evacuated the injured fighters last night."

Dibeh Fakhri,a spokesman for the International Committee of the Red Cross, called the situation in Bani Walid and Sirte "deteriorating."

"We have not been able to enter yet," Fakhri said. "We have surgical and medical supplies ready in Misrata in case we are able to deliver them into Sirte shortly. Our teams have positioned supplies in the town of Tarhuna close to Bani Walid but we have not been able to contact the authorities to be able to enter the town itself. Our teams are monitoring the situation very closely."